At the beginning of the story, Percy Jackson believes he is a troubled youth who suffers from ADHD and dyslexia. After he loses his mother in a battle with the Minotaur, Percy finds himself at Camp Half-Blood. Here, Percy learns that the gods he once believed to be myths are real. He also learns from Annabeth that the gods often have children with humans. These children are known as demigods. Annabeth explains to Percy that Camp Half-Blood is the safest place for demigods. Percy also learns that his characteristics of ADHD are a result of his "battlefield reflexes." In addition, Percy discovers that his difficulty in reading is due to his brain being "hardwired for ancient Greek."

When Percy first arrives at camp, he is labeled as "undetermined." This means that he is a demigod whose parent has not yet been identified. Percy stays in the cabin with the children of Hermes. However, it soon becomes evident that Percy is the son of Poseidon. As a result, Percy relocates to cabin three, which is reserved for the children of Poseidon. Percy is its only inhabitant.